1. Field of the Invention
The present invention relates to a data recording system that uses a memory having a limitation on the number of writes.
2. Description of the Related Art
Memories for saving data are in widespread use in a variety of fields. Especially, the capacities of nonvolatile semiconductor memories (such as a flash memory, an EEPROM, a PROM, etc.) have been increased, and their intended uses have expanded in recent years. Semiconductor memories are normally superior in resistance to vibration, and low power consumption compared with other nonvolatile memories such as a magnetic disk, etc.
However, a limit number of writes (the number of erase/write cycles) of a nonvolatile semiconductor memory is normally small compared with a magnetic disk, etc. For this reason, the number of writes of a nonvolatile semiconductor memory can reach the limit number if the data of the memory is frequently updated for a long period. By way of example, if the limit number of writes is 100,000, a nonvolatile semiconductor memory must be replaced after approximately four years use in a system in which the status information of a network is written to the nonvolatile semiconductor memory at time intervals of 15 minutes.
Patent Documents 1 to 3 recite the techniques for extending the lifetime of a nonvolatile memory. Namely, Patent Document 1 (Japanese Published Unexamined Patent Application No. 2000-339506) recites the procedures for writing data to a nonvolatile memory only if the obtained data exceeds a predetermined threshold value in a management system that saves data representing the state of an image forming apparatus. Patent Document 2 (Japanese Published Unexamined Patent Application No. H8-125914) recites the configuration in which a location to be written is switched to another area within a nonvolatile memory if the number of writes to one block of the memory exceeds a predetermined number. Patent Document 3 (Japanese Published Unexamined Patent Application No. H7-281842) recites the configuration in which new data is written to a ferroelectric memory if the number of writes to a flash memory exceeds a predetermined number in a storage device having the flash memory and the ferroelectric memory.
However, the above described conventional techniques have problems to be overcome. Namely, the configuration recited in Patent Document 1 cannot be adopted for a system that requires entire data to be stored, because part of collected data is discarded without being saved in a memory. With the configuration recited in Patent Document 2, the capacity of a memory must be made large, leading to an increase in the cost. Additionally, with the configuration recited in Patent Document 3, the lifetime of a flash memory itself is not extended.